Piezoelectric sensor configuration for detecting damage in a structure

ABSTRACT

Described herein is an apparatus for detecting damage in a structure that includes a plurality of first piezoelectric sensing elements arranged in a generally circular shape. The apparatus also includes an annular-shaped second piezoelectric sensing element positioned adjacent the plurality of first piezoelectric sensing elements. One of the plurality of first piezoelectric sensing elements or the annular-shaped second piezoelectric sensing element generates a wave through the structure and other of the plurality of first piezoelectric sensing elements or the annular-shaped second piezoelectric sensing element senses the wave after passing through the structure.

FIELD

This disclosure relates generally to detecting damage in a structure,and more particularly to detecting cracks in a structure using apiezoelectric sensor configuration.

BACKGROUND

Structures experiencing loads or exposed to various environmentalfactors are susceptible to damage, such as cracking, corrosion,delamination, and the like. Damage to structures may lead to aestheticflaws, structural degradation, inefficiencies, poor performance, andeven catastrophic failure. Accordingly, the detection of damage tostructures may be desirable to mitigate or prevent the occurrence ofsuch negative consequences. In some circumstances, the negativeconsequences of damage to the structure can be mitigated or preventedthrough detection and repair of the damage.

Some structures include features that are particularly susceptible todamage or the inducement of damage. For example, cracks tend to form atand emanate from fastener holes in surfaces of certain structures, suchas aircraft.

SUMMARY

The subject matter of the present application has been developed inresponse to the present state of the art, and in particular, in responseto the problem of, and the need to, detect damage, such as crackformations, in various structures, such as aircraft, that have not yetbeen fully solved by currently available techniques. Once disassembled,conventional detection methods, such as non-destructive ultrasonictesting techniques, may be used to detect such damage. The time andeffort associated with disassembly, ultrasonic testing, and reassemblyof some structures using conventional techniques can be overlyburdensome in both time and cost. In contrast, some detection techniquesuse an arrangement of piezoelectric elements positioned in situ about ahole formed in the structure. However, such conventional in situtechniques rely on a rectangular or side bank arrangement ofpiezoelectric elements relative to the hole, which fails to provide anadequate or comprehensive detection of damage in the structure proximatethe hole. Accordingly, the subject matter of the present application hasbeen developed to provide an apparatus, system, and method for detectingdamage in a structure that overcome at least some of the above-discussedshortcomings of prior art techniques.

According to one embodiment, an apparatus for detecting damage in astructure includes a plurality of first piezoelectric sensing elementsarranged in a generally circular shape. The apparatus also includes anannular-shaped second piezoelectric sensing element positioned adjacentthe plurality of first piezoelectric sensing elements. One of theplurality of first piezoelectric sensing elements or the annular-shapedsecond piezoelectric sensing element generates a wave through thestructure. The one of the plurality of first piezoelectric sensingelements or the annular-shaped second piezoelectric sensing element caninclude all (or one or some) of the first piezoelectric sensing elementsor the annular-shaped second piezoelectric sensing element. Other of theplurality of first piezoelectric sensing elements or the annular-shapedsecond piezoelectric sensing element senses the wave after passingthrough the structure. The other of the plurality of first piezoelectricsensing elements or the annular-shaped second piezoelectric sensingelement can include all (or one or some) of the plurality of firstpiezoelectric sensing elements or the annular-shaped secondpiezoelectric sensing element.

In certain implementations of the apparatus, the annular-shaped secondpiezoelectric sensing element surrounds the plurality of firstpiezoelectric sensing elements.

According to some implementations, the apparatus also includes aplurality of third piezoelectric sensing elements arranged in agenerally circular shape surrounding the annular-shaped secondpiezoelectric sensing element. Additionally, the apparatus may includean annular-shaped fourth piezoelectric sensing element that surroundsthe plurality of third piezoelectric sensing elements. In oneimplementation, the plurality of first piezoelectric sensing elementsincludes at least five piezoelectric sensing elements.

In some implementations of the apparatus, the plurality of firstpiezoelectric sensing elements and the annular-shaped secondpiezoelectric sensing element are bonded onto the structure. Theapparatus may include a plurality of electrical lead pairs bonded to thestructure. Each of the plurality of electrical lead pairs includes atleast two electrical leads electrically coupled to a respective one ofthe first and second piezoelectric sensing elements.

According to one implementation, the apparatus includes a mobile testhead that is movable along the structure. The mobile test head includesthe plurality of first piezoelectric sensing elements and theannular-shaped second piezoelectric sensing element.

In certain implementations of the apparatus, the plurality of firstpiezoelectric sensing elements and the annular-shaped secondpiezoelectric sensing element collectively define a first sensingelement group. The apparatus also includes a second sensing elementgroup that is spaced apart from the first sensing element group. Thesecond sensing element group includes a plurality of third piezoelectricsensing elements arranged in a generally circular shape and anannular-shaped fourth piezoelectric sensing element positioned adjacentthe plurality of second piezoelectric sensing elements. One of theplurality of first piezoelectric sensing elements and the annular-shapedsecond piezoelectric sensing element generates a wave through thestructure and one of the plurality of third piezoelectric sensingelements and the annular-shaped fourth piezoelectric sensing elementsenses the wave after passing through the structure. The first sensingelement group can be centered around a first hole formed in thestructure and the second sensing element group can be centered around asecond hole formed in the structure. The apparatus may also include acontroller that is configured to selectively switch between operation ina first mode and second mode. In the first mode, the controlleractivates the first sensing element group to generate a first wave andreceives input from the first sensing element group regardingcharacteristics of the first wave and activates the second sensingelement group to generate a second wave and receives input from thesecond sensing element group regarding characteristics of the secondwave. In the second mode, the controller activates the first sensingelement group to generate a third wave and receives input from thesecond sensing element group regarding characteristics of the thirdwave.

According to some implementations, the apparatus further includes acontroller that is configured to activate the one of the plurality offirst piezoelectric sensing elements or the annular-shaped secondpiezoelectric sensing element to generate the wave, and to receive inputfrom other of the plurality of first piezoelectric sensing elements orthe annular-shaped second piezoelectric sensing element that sense thewave after passing through the structure. The apparatus can also includea plurality of electrical lead pairs bonded to the structure. Each ofthe plurality of electrical lead pairs can include at least twoelectrical leads electrically coupled to a respective one of the firstand second piezoelectric sensing elements. The controller includes anelectrical lead interface that interfaces with the plurality ofelectrical lead pairs to send electrical signals to or receiveelectrical signals from the plurality of first piezoelectric sensingelements and the annular-shaped second piezoelectric sensing element.

In certain implementations, the annular-shaped second piezoelectricsensing element is substantially coaxial with the generally circularshape of the plurality of first piezoelectric sensing elements. Theannular-shaped second piezoelectric sensing element can be positionedradially outwardly away from the plurality of first piezoelectricsensing elements. According to some implementations, each of the firstpiezoelectric sensing elements includes a piezoelectric sensor disc.

According to another embodiment, a system includes a structure with ahole. The system also includes a plurality of first piezoelectricsensing elements bonded to the structure and arranged in a generallycircular shape about the hole. Additionally, the system includes anannular-shaped second piezoelectric sensing element bonded to thestructure. The annular-shaped second piezoelectric sensing element ispositioned adjacent the plurality of first piezoelectric sensingelements and about the hole. One of the plurality of first piezoelectricsensing elements or the annular-shaped second piezoelectric sensingelement generates a wave through the structure and other of theplurality of first piezoelectric sensing elements or the annular-shapedsecond piezoelectric sensing element senses the wave after passingthrough the structure adjacent the hole.

In some implementations of the system, each of the plurality of firstpiezoelectric sensing elements is positioned an equal distance away froma center of the hole, and the annular-shaped piezoelectric sensingelement is coaxial with the hole. The structure can include an aircraftin certain implementations.

According to certain implementations of the system, the plurality offirst piezoelectric sensing elements and the annular-shaped secondpiezoelectric sensing element collectively define a first sensingelement group and the hole is a first hole. The structure may furtherinclude a second hole that is spaced apart from the first hole. Thesystem further includes a second sensing element group that includes aplurality of third piezoelectric sensing elements bonded to thestructure and arranged in a generally circular shape about the secondhole and an annular-shaped fourth piezoelectric sensing element bondedto the structure and being adjacent the plurality of third piezoelectricsensing elements. One of the plurality of first piezoelectric sensingelements and the annular-shaped second piezoelectric sensing elementgenerates a wave through the structure and one of the plurality of thirdpiezoelectric sensing elements and the annular-shaped fourthpiezoelectric sensing element senses the wave after passing through thestructure.

In yet another embodiment, a method for detecting damage in a structureincludes generating a wave through the structure from one of a pluralityof first piezoelectric sensing elements arranged in a generally circularshape or an annular-shaped second piezoelectric sensing element. Themethod also includes sensing the wave after passing through thestructure at other of the plurality of first piezoelectric sensingelements or the annular-shaped second piezoelectric sensing element.

The described features, structures, advantages, and/or characteristicsof the subject matter of the present disclosure may be combined in anysuitable manner in one or more embodiments and/or implementations. Inthe following description, numerous specific details are provided toimpart a thorough understanding of embodiments of the subject matter ofthe present disclosure. One skilled in the relevant art will recognizethat the subject matter of the present disclosure may be practicedwithout one or more of the specific features, details, components,materials, and/or methods of a particular embodiment or implementation.In other instances, additional features and advantages may be recognizedin certain embodiments and/or implementations that may not be present inall embodiments or implementations. Further, in some instances,well-known structures, materials, or operations are not shown ordescribed in detail to avoid obscuring aspects of the subject matter ofthe present disclosure. The features and advantages of the subjectmatter of the present disclosure will become more fully apparent fromthe following description and appended claims, or may be learned by thepractice of the subject matter as set forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the advantages of the subject matter may be more readilyunderstood, a more particular description of the subject matter brieflydescribed above will be rendered by reference to specific embodimentsthat are illustrated in the appended drawings. Understanding that thesedrawings depict only typical embodiments of the subject matter and arenot therefore to be considered to be limiting of its scope, the subjectmatter will be described and explained with additional specificity anddetail through the use of the drawings, in which:

FIG. 1 is a top plan view of an aircraft showing a detailed section of asurface of the aircraft having piezoelectric sensing elements accordingto one embodiment;

FIG. 2 is a perspective view of a surface of a structure havingpiezoelectric sensing elements according to one embodiment;

FIG. 3 is a top plan view of a group of piezoelectric sensing elementsaccording to one embodiment;

FIG. 4 is a top plan view of another group of piezoelectric sensingelements each electrically coupleable to a controller according to oneembodiment;

FIG. 5 is a top plan view of first and second groups of piezoelectricsensing elements spaced apart on a surface of a structure according toone embodiment;

FIG. 6 is a schematic block diagram of a controller for controlling agroup of piezoelectric sensing elements according to one embodiment;

FIG. 7 is a schematic flow diagram of a method for detecting damage in astructure according to one embodiment; and

FIG. 8 is a perspective view of a mobile test head having a group ofpiezoelectric sensing elements, with the test head being positionedrelative to a surface of a structure, according to one embodiment.

DETAILED DESCRIPTION

Reference throughout this specification to “one embodiment,” “anembodiment,” or similar language means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment of the present disclosure.Appearances of the phrases “in one embodiment,” “in an embodiment,” andsimilar language throughout this specification may, but do notnecessarily, all refer to the same embodiment. Similarly, the use of theterm “implementation” means an implementation having a particularfeature, structure, or characteristic described in connection with oneor more embodiments of the present disclosure, however, absent anexpress correlation to indicate otherwise, an implementation may beassociated with one or more embodiments.

Referring to FIG. 1, one embodiment of an aircraft 10 is shown. Thedepicted aircraft 10 includes a body 12 or fuselage, a pair of wings 14coupled to and extending from the body 12, a vertical stabilizer 16coupled to the body, and a pair of horizontal stabilizers 18 coupled tothe body and/or the vertical stabilizer. The aircraft 10 includesfeatures representative of a commercial passenger or military transportaircraft. However, the aircraft 10 can be any of various other types ofcommercial or non-commercial aircraft, such as personal aircraft,fighter jets, helicopters, spacecraft, and the like. Moreover, althoughan aircraft is depicted in the illustrated embodiment, in otherembodiments, another structure, such as a vehicle (e.g., boat,automobile, etc.) or non-mobile complex structure (e.g., building,bridge, machinery, etc.) can be used without departing from the essenceof the present disclosure.

Generally, the body 12, wings 14, vertical stabilizer 16, and horizontalstabilizers 18 of the aircraft 10 each includes an internal frameenveloped by a cover or skin. The cover is coupled to the frame to forman exterior shell of the aircraft. Most commonly, the cover is coupledto the frame using a plurality of fasteners that extend through holes inthe cover and engage the frame. The aircraft 10 or other structure mayinclude additional interior layers or structures with holes formedtherein to receive fasteners for coupling other components, layers, orstructures. Accordingly, although in the depicted embodiment, a hole 30is shown formed in an outer surface 13 of the body 12 of the aircraft 10and a sensing element group 20 is shown coupled onto the outer surface13, in other embodiments, the hole can be formed in an outer or innersurface of another portion of the aircraft, or other structure, and thesensing element group can be coupled onto the outer or inner surface ofthat portion.

The aircraft 10 may include tens of thousands of holes 30 and associatedfasteners in the various portions, components, and sub-structures of theaircraft. The areas adjacent or proximate the holes 30 are susceptibleto damage, such as cracking, by virtue of experiencing loads and beingexposed to corrosive environmental factors. Although the presentdisclosure includes apparatus, systems, and methods for detecting damagein any of various structures, such as areas around any holes in thestructure, regardless of susceptibility to damage, in some embodiments,the present disclosure is configured to target areas in structures thatmay be more susceptible to damage than other areas. For example, in someembodiments, only the holes 30 of the aircraft 10 particularlysusceptible to damage are monitored for damage using the apparatus,systems, and methods of the present disclosure.

According to one embodiment, the sensing element group 20 includes aplurality of first piezoelectric sensing elements 22 and a secondpiezoelectric sensing element 24. The sensing element group 20 iscoupled to an outer surface 13 of the body 12 of the aircraft 10. Theouter surface 13 can be an outer or exterior surface, or an inner orinterior surface of the body 12. Again, although a surface of the bodyof an aircraft is being shown, the surface of any other portion (e.g.,wings, stabilizers, etc.) of the aircraft or other structure is equallyapplicable. The piezoelectric sensing elements 22, 24 can be coupled tothe outer surface 13 of the body 12 using any of various couplingtechniques. For example, in some implementations, such as FIG. 2 (whichshows features analogous to the features of FIG. 1 with like numbersreferring to like elements), the piezoelectric sensing elements 122, 124are bonded onto the surface 113 of the body 112 using a bonding adhesiveor other bonding technique. In certain implementations, the aircraft 10may have additional layers that overlay or are coated onto the outersurface 13 of the body 12 and the piezoelectric sensing elements.Accordingly, in such implementations, the piezoelectric sensing elements22, 24 can be considered embedded within the body 12. Although lesspreferable, in some implementations, the piezoelectric sensing elements22, 24 may be removable from the outer surface 13, and thus may beindirectly and selectively coupled to the surface during periods oftesting.

Each of the piezoelectric sensing elements 22, 24 is made from any ofvarious piezoelectric materials. As defined herein, a piezoelectricmaterial is any solid material that accumulates an electric charge whendeformed, and deforms when subject to an electric charge. In otherwords, not only is a piezoelectric material capable of accumulating anelectric charge when subject to a force or load that deform or otherwisechange the dimensions of the piezoelectric material, but also is capableof changing dimensions to generate a force or load when an electricfield is applied to the material. Additionally, piezoelectric materialscan include materials that experience deformation when subject toelectric and magnetic energy fields, such as electrostrictive andmagnetostrictive materials, respectively, but may not accumulatecorresponding energy fields when subject to deformation. In this manner,each of the piezoelectric sensing elements 22, 24 can be considered ordefined as a transducer. The force or load can be acoustic waves (e.g.,lamb waves or elastic waves) that propagate through the body 12 andalong the outer surface 13 of the body 12. Accordingly, depending on theconfiguration of operating mode, a piezoelectric sensing element can actas either an electric accumulator of electrical charge for sensingacoustic waves in the body 12 and along the outer surface 13 of thebody, or a wave generator that generates acoustic waves through the bodyand along the surface of the body.

Generally, the power of the accumulated (sensed) electric charge isdirectly proportional to the magnitude of the change in dimension causedby an acoustic wave received at the piezoelectric sensing element, thuspiezoelectric sensing elements operating as an electric accumulator areable to detect characteristics (e.g., amplitude, frequency, etc.) ofreceived acoustic waves. The inverse is also true, which is that thecharacteristics of acoustic waves generated by a piezoelectric sensingelement are directly proportional to the power of the electrical chargeapplied to the piezoelectric sensing element, thus piezoelectric sensingelements operating as a wave generator are able to generate acousticwaves with controlled characteristics.

The sensing element group 20 includes a threshold number of firstpiezoelectric sensing elements 22 or transducers arranged in a generallycircular shape. Each piezoelectric sensing element 22 can have any ofvarious shapes and sizes. In the illustrated embodiments, eachpiezoelectric sensing element 22 has a generally circular, disc-likeshape. However, in other embodiments, each piezoelectric sensing element22 can have a non-circular shape. In some embodiments, the thresholdnumber is at least five. For example, in the illustrated implementation,the sensing element group 20 includes six first piezoelectric sensingelements 22. In other implementations, the sensing element group 20includes more than six piezoelectric sensing elements 22 arranged in agenerally circular shape (see, e.g., FIG. 2). As defined herein,piezoelectric sensing elements 22 are arranged in a generally circularshape when each of the threshold number of piezoelectric sensingelements is positioned a substantially equal distance radially away froma common center point such that a single circle (see, e.g., circle 221of FIG. 3) extends through an approximate center-point of each element.

The second piezoelectric sensing element or transducer 24 has agenerally annular shape. As defined herein, an annular shape can be agenerally ring-like shape or a shape forming a continuous ring. In otherwords, the second piezoelectric sensing element 24 forms a continuousshape that encloses or encircles an open space. In the illustratedembodiments, the second piezoelectric sensing element 24 is circularsuch that the second piezoelectric sensing element forms a circular ringor band. However, in other embodiments, the second piezoelectric sensingelement 24 is non-circular (e.g., ovular, elliptical, square,rectangular, triangular, etc.) to form a non-circular annular ring.Additionally, an annular shape can be defined as forming a portion of aring or circle. For example, an annular shape need not be a continuousring in some implementations, but rather can include one or more curvedportions that form an entire ring or a portion of a ring.

Additionally, the second piezoelectric sensing element 24 of the sensingelement group 20 is positioned adjacent the first piezoelectric sensingelements 22 of the same group. As defined herein, adjacent meansproximate, near, or next to in one embodiment. According to someimplementations, as shown in FIG. 1, the second piezoelectric sensingelement 24 is positioned radially outward of the first piezoelectricsensing elements 22 to surround the first piezoelectric sensingelements. In other implementations, the second piezoelectric sensingelement 24 is positioned radially inward of the first piezoelectricsensing elements such that the first piezoelectric sensing elementssurround the second piezoelectric sensing element.

In those embodiments with a circular annular-shaped second piezoelectricsensing element 24, the second piezoelectric sensing element 24 can bepositioned on the outer surface 13 such that the element 24 is coaxialor centrally aligned with the generally circular shape of the firstpiezoelectric sensing elements 22. In this manner, the secondpiezoelectric sensing element 24 is positioned an equal distanceradially away from each of the first piezoelectric sensing elements 22.

Referring to FIG. 3, a sensing element group of the present disclosurecan include multiple sets of first and second piezoelectric sensingelements. For example, in the illustrated embodiment, the sensingelement group 220 (which shows features analogous to the features ofFIG. 1 with like numbers referring to like elements) includes a firstset of first and second piezoelectric sensing elements 222, 224 and asecond set of first and second piezoelectric sensing elements 226, 228positioned radially outward away from the first set. Similar to thefirst set of piezoelectric sensing elements 222, 224, the second set ofpiezoelectric sensing elements 226, 228 includes a plurality of firstpiezoelectric sensing elements 226 arranged in a generally circularshape, and an annular-shaped second piezoelectric sensing element 228positioned radially outwardly from the first piezoelectric sensingelements 226. However, while the first set of piezoelectric sensingelements 222, 224 includes six piezoelectric sensing elements 222, thesecond set of piezoelectric sensing elements 226, 228 includes eightpiezoelectric sensing elements 226. In one embodiment, as shown, thegenerally circular shapes of the first piezoelectric sensing elements222, 226, and the annular-shaped second piezoelectric sensing elements224, 228, of the first and second sets are coaxial or centrally alignedwith each other.

Referring to FIG. 4, each sensing element group includes a plurality ofelectrical lead pairs. Each of the electrical lead pairs is electricallycoupled to a respective one of the first and second piezoelectricsensing elements. Furthermore, each electrical lead pair includes twoelectrical leads (e.g., a positive lead and a negative lead) that areelectrically coupled together via the corresponding piezoelectricsensing element. In other words, the piezo material of a piezoelectricsensing element is positioned between the positive lead and negativelead of the electrical lead pair. When a voltage is transmitted acrossthe positive lead and negative lead of a pair, the correspondingpiezoelectric sensing element strains to produce an acoustic wave.

According to an exemplary embodiment, the sensing element group 20 has aplurality of electrical lead pairs 40 each electrically coupled to arespective one of the first piezoelectric sensing elements 22, and anelectrical lead pair 42 electrically coupled to the second piezoelectricsensing element 24. Each electrical lead pair 40, 42 can include twoleads, which can be strips, lines, conduits, or the like, made from anelectrically conductive material, such as copper, aluminum, etc. In someimplementations, the leads of the electrical lead pairs 40, 42 may bewrapped or covered in non-conductive insulation, such as a polymersleeve, to form a sheathed cable. The electrical lead pairs 40, 42 arecoupled to the outer surface 13 of the body 12. In some implementations,the electrical lead pairs 40, 42 are bonded to the outer surface 13using any of various bonding techniques, such as the same bondingtechnique used to bond the first and second piezoelectric sensingelements 22, 24 to the surface. The electrical lead pairs 40 may overlayor underlay the second piezoelectric sensing element 24 while remainingelectrically isolated from the second piezoelectric sensing element.Additionally, in certain implementations, the aircraft 10 may haveadditional layers that overlay or are coated onto the outer surface 13of the body 12 and at least a portion of the electrical lead pairs 40,42.

The sensing element group 20 also includes an electrical lead interface44. Each electrical lead pair 40, 42 extends from a first end coupled torespective first and second piezoelectric sensing elements to a secondend forming at least part of the electrical lead interface 44. Incertain implementations, the electrical lead interface 44 includes anelectrical connector that is electrically coupled to the second ends ofthe leads of the electrical lead pairs 40, 42. In some implementations,the electrical lead interface 44 is a platform or area that contains thesecond ends of the leads of the electrical lead pairs 40, 42 in a fixedor uniform manner. The electrical lead interface 44, and in someimplementations the second ends of the leads of the electrical leadpairs 40, 42 themselves, are physically accessible in situ by a user. Inimplementations where the outer surface 13 is an outer surface, theelectrical lead interface 44 and electrical lead pairs 40, 42 arephysically accessible in situ by a user external to the outer surface.Similarly, where the outer surface 13 is an inner surface, theelectrical lead interface 44 and electrical lead pairs 40, 42 arephysically accessible in situ by a user internal to the inner surface.The electrical lead interface 44 may be fitted with a removable ormovable cover that can be engaged to access the electrical leadinterface 44.

Referring again to FIG. 4, the sensing element group 20 can form part ofa damage detection system 90. The damage detection system 90 alsoincludes a controller 50 that is electrically communicable with thesensing element group 20 via an electrical lead interface 46 andelectrical communication line 48 or wire. The electrical lead interface46 interfaces with the plurality of electrical lead pairs 40, 42directly or through the electrical lead interface 44. For example, inone implementation, the electrical lead interface 46 may include aconnector that interfaces with a connector of the electrical leadinterface 44. When interfaced with the plurality of electrical leadpairs 40, 42, the controller 50 can send electrical signals to andreceive electrical signals from the first and second piezoelectricsensing elements 22, 24 via the electrical communication line 48.

As shown in FIG. 6, according to one embodiment, the controller 50includes an emitting transducer module 70, a receiving transducer module72, and a detection mode module 74. The emitting transducer module 70 isconfigured to generate a transducer command 76 that commands anelectrical pulse with characteristics corresponding to an acoustic wavewith desired characteristics be sent to the one or more piezoelectricsensing elements acting as the wave generator. The transducer command 76may be sent to an electrical pulse generator that is separate orintegral with the controller 50 that generates the desired electricalpulse(s), and transmits the electrical pulse(s) to the wave generatingpiezoelectric sensing element(s), in response to receiving thetransducer command 76. The electrical pulse(s) are sent to the wavegenerating piezoelectric sensing element(s) from the generator via theelectrical communication line 48 and electrical lead(s) associated withthe wave generating piezoelectric sensing element(s).

In some embodiments, the annular-shaped second piezoelectric sensingelement 24 acts as the wave generator. Accordingly, in response toreceiving the transducer command 76, the electrical pulse generatortransmits electrical pulses to the second piezoelectric sensing element24. Correspondingly, in response to receiving the electrical pulses fromthe electrical pulse generator, the second piezoelectric sensing element24 deforms to generate an acoustic wave with the desired characteristicsthat propagates through the body 12 and along the outer surface 13 ofthe body. Because the second piezoelectric sensing element 24 has anannular shape, the acoustic wave generated by the second piezoelectricsensing element more uniformly and broadly propagates through the body12 and along the outer surface 13 with reduced attenuation compared tomultiple spaced-apart piezoelectric sensing discs.

The receiving transducer module 72 is configured to receive transducerinput 78 from the one or more piezoelectric sensing elements acting asthe electric accumulator. As discussed above, upon deformation due tothe exposure to acoustic waves, the piezoelectric sensing elementsaccumulate electrical charge that is approximately proportional to themagnitude of the acoustic waves. The transducer input 78 includessignals representative of the accumulated electrical charge, and thusrepresentative of the magnitude of the acoustic waves. The transducerinput 78 is communicated from the piezoelectric sensing elements to thecontroller 50 via the electrical lead(s) associated with the one or morepiezoelectric sensing elements acting as the electric accumulator andthe electrical communication line 48.

In some embodiments, the plurality of first piezoelectric sensingelements 22 arranged in the generally circular shape act as the electricaccumulators. Accordingly, after being exposed to acoustic wavesgenerated by the annular-shaped second piezoelectric sensing element 24,the plurality of first piezoelectric sensing elements 22 generate andtransmit transducer input 78 back to the controller 50. Because theacoustic waves are generated by an annular-shaped second piezoelectricsensing element 24, and the first piezoelectric sensing elements 22 arearranged in a generally circular shape corresponding to the shape of thesecond piezoelectric sensing element, the first piezoelectric sensingelements monitor and detect a wider portion (e.g., area) of the body 12and outer surface 13 compared to traditional approaches. Moreover,positioning the second piezoelectric sensing elements 22 uniformly abouta point and equidistant from that point, such as in a circulararrangement, promotes accuracy and broader damage detection coverage.

According to some embodiments, the receiving transducer module 72, orseparate analysis module (not shown) utilizes the transducer input 78 todetect the presence of damage in the structure (e.g., the body 12 and/orthe outer surface 13). The receiving transducer module 72 can use any ofvarious methods and/or apply any of various algorithms for detectingdamage based on the transducer input 78. In certain embodiments, thetransducer module 72 detects damage by applying the transducer input 78to a baseline-less model without relying on predetermined or knownbaselines.

However, in yet some embodiments, the transducer module 72 detectsdamage by applying the transducer input 78 to a baseline model byrelying on predetermined or known baseline waveforms. For example, inone embodiment, the receiving transducer module 72 compares thetransducer input 78 with an expected transducer input or baseline inputto detect the presence of damage in the structure. The expectedtransducer input represents the input expected to be received inresponse to hypothetical acoustic waves with specific characteristicspassing through a structure without damage. Accordingly, variations inthe actual transducer input 78 compared to the expected transducer inputindicates abnormalities or damage (e.g., cracking) in the structure. Fora proper comparison, the transducer command 76 generated by the emittingtransducer module 70 commands the annular-shaped second piezoelectricsensing element 24, or wave generator, to generate in the structureactual acoustic waves with characteristics matching the hypotheticalacoustic waves.

Although, in the above embodiment, the plurality of first piezoelectricsensing elements 22 each act as an electrical accumulator and theannular-shaped second piezoelectric sensing element 24 acts as the wavegenerator, in other embodiments, the plurality of first piezoelectricsensing elements 22 each may act as a wave generators and the secondpiezoelectric sensing element 24 may act as the electrical accumulator.

As discussed above, various portions, components, and sub-structures ofa structure can be more susceptible to damage than others. Accordingly,in certain embodiments, a sensing element group 20 can be positionednear or adjacent a susceptible portion of a structure to more accuratelyand comprehensively detect damage on or near the portion. For example,many holes formed in structures are particularly susceptible to cracksthat form at and emanate from the holes. For this reason, in theillustrated embodiments, the sensing element group 20 is positioned nearor adjacent the hole 30 formed in the body 12 of the aircraft 10. Morespecifically, and for enhanced results, the sensing element group 20 ispositioned concentrically around the hole 30 such that centers of thecircular shape of the first piezoelectric sensing elements 22 andannular-shaped second piezoelectric sensing element 24 are centrallyaligned or coaxial with a central axis of the hole 30 (see, e.g.,central axis 231 of the hole 230 shown in FIG. 3). In thisconfiguration, each of the first piezoelectric sensing elements 22 arepositioned an equal distance radially away from the central axis of thehole, and the peripheries of the second piezoelectric sensing element 24is spaced the same radial distance away from the central axis of thehole. Moreover, in this configuration, the sensing element group 20 ispositioned to more uniformly, broadly, and accurately detect damage,including cracks, in the immediate space around the hole 30.

Although the above embodiment is described and shown with reference tothe position of the sensing element group 20 around a hole 30 formed inan aircraft 10 to detect damage around the hold, in other embodiments,the sensing element group can be placed about another portion,component, or sub-structure of an aircraft or other structure to detectdamage about the portion, component, or sub-structure in the same orsimilar manner.

Additionally, in certain embodiments, multiple, spaced-apart, sensingelement groups can be configured to detect not only damage in thestructure about which each sensing element group is positioned, but alsodamage in the structure between the sensing element groups. For example,as shown in FIG. 5, a damage detection system 92 includes at least twosensing element groups 20A, 20B each surrounding respective holes 30A,30B formed in the outer surface 13. Each sensing element group 20A, 20Bis configured to individually and independently detect damage in thestructure (e.g., body 12 and outer surface 13) about the respectiveholes 30A, 30B (e.g., cracks emanating from the holes) as describedabove.

As shown, the damage detection system 92 includes a controller 60 andseparate electrical lead interfaces 46 and electrical communicationlines 48 that separately facilitate the communication between thesensing element groups 20A, 20B, respectively, and the controller 60.The controller 60 has features analogous to the features of thecontroller 50 of FIG. 6. More specifically, the controller 60 includesan emitting transducer module 70 and receiving transducer module 72 thatseparately control the generation and detection of acoustic waves foreach sensing element group 20A, 20B in a first mode of operation.However, in a second mode of operation, the emitting transducer module70 controls the generation of acoustic waves 77 from one of the sensingelement groups 20A, 20B, and the receiving transducer module 72 controlsthe detection of the generated acoustic waves by the other of thesensing element groups. For example, in the illustrated embodiment, theemitting transducer module 70 controls the sensing element group 20A togenerate the acoustic waves 77, and the receiving transducer module 72controls the sensing element group 20B to receive the acoustic waves 77.The emitting transducer module 70 may activate the second piezoelectricsensing element 24 of the sensing element group 20A to generate theacoustic waves, activate one or more of the first piezoelectric sensingelements 22 of the sensing element group 20A to generate the acousticwaves, or activate both the first and second piezoelectric sensingelements of the sensing element group 20A to generate the acousticwaves.

The receiving transducer module 72 of the controller 60, or separateanalysis module, then applies the transducer input 78 from the sensingelement group 20B according to one of various methods to detect thepresence of damage in the portion of the structure between the holes30A, 30B. According to one implementation, the receiving transducermodule 72 compares the transducer input 78 from the sensing elementgroup 20B with an expected transducer input to detect the presence ofdamage in the portion of the structure between the holes 30A, 30B. Inthis implementation, the expected transducer input in the second moderepresents the input expected to be received in response to hypotheticalacoustic waves with specific characteristics passing through a portionof the structure between the holes 30A, 30B that does not have damage.For a proper comparison, the transducer command 76 generated by theemitting transducer module 70 commands the sensing element group 20A,which acts as the wave generator, to generate in the structure betweenthe holes 30A, 30B actual acoustic waves with characteristics matchingthe hypothetical acoustic waves.

In some embodiments, the controller 60 includes a detection mode module74 that selectively switches operation of the emitting transducer module70 and receiving transducer module 72 between the first and secondmodes. According to certain implementations, the detection mode module74 may operate the modules 70, 72 first in the first mode to detectdamage near and just around certain features in the structure, such asthe holes 30A, 30B, and then switch to operation in the second mode todetect damage in the structure between the features or holes.Alternatively, the detection mode module 74 may operate the modules 70,72 first in the second mode to detect damage in the structure betweenthe features or holes, and then in the first mode to detect damage nearand just around the certain features in the structure. Also, in someimplementations, the detection mode module 74 may operate the modules70, 72 in only the first or second mode as desired.

Although the above embodiment shown in relation to FIG. 5 shows onesensing element group acting as a wave generator and one sensing elementgroup acting as an acoustic wave receiver or detector, in otherembodiments, a system can have more than one sensing element groupacting as wave generators and more than one sensing element group actingas wave detectors.

Referring to FIG. 7, according to one embodiment, a method 300 fordetecting damage in a structure can be executed by the systems andapparatus described herein, or other systems and apparatus. The method300 begins by determining whether interstitial detection is desired at302. Interstitial detection can be defined as the detection of damage instructure between sensing element groups, such as described and shown inrelation to the damage detection system 92 of FIG. 5. If interstitialdetection is not desired at 302 (i.e., detection of damage in theimmediate feature (e.g., hole) about which a sensing element groupsurrounds is desired), then the method 300 proceeds to generate acousticwaves from one of the annular-shaped transducer (e.g., sensing element)or plurality of circularly-arranged transducers of a sensing elementgroup at 304. Additionally, the method 300 receives or detects thegenerated acoustic waves at other of the annular-shaped transducer(e.g., sensing element) or plurality of circularly-arranged transducersof the sensing element group at 306. Then, the method 300 determinesmaterial conditions (e.g., damage) proximate or immediately surroundingthe structure based on the characteristics of the received waves at 308.

However, if interstitial detection is desired at 302, then the method300 proceeds to generate acoustic waves from a first transducer group(e.g., first sensing element group) at 310, and receives or detects thegenerated acoustic waves at a second transducer group at 312. The secondtransducer group is spaced apart from the first transducer group. Themethod 300 also includes determining material conditions in thestructure between the first and second transducer groups (e.g., betweenthe first and second features about which the first and secondtransducer groups surround) at 314.

Referring now to FIG. 8, a sensing element group 420 can be integratedinto a mobile test head 400 of a damage detecting system. The sensingelement group 420 includes a plurality of first piezoelectric sensingelements 422 arranged in a circular shape on a scanning surface 402 ofthe mobile test head 400. Additionally, the sensing element group 420includes an annular-shaped second piezoelectric sensing element 424 onthe scanning surface 402 of the head. The annular-shaped secondpiezoelectric sensing element 424 is positioned around and concentricwith the circular shape of the first piezoelectric sensing elements 422in a manner as described above in relation to the sensing element group20 of FIG. 1. The scanning surface 402 of the mobile test head 400 canbe brought into at least close proximity with a surface 413 of astructure 412 as shown by the directional arrow. In one implementation,the scanning surface 402 is placed in contact with the surface 413. Thestructure 412 includes at least one hole 430 or other feature ofinterest formed in the surface 413 of the structure.

In operation, the mobile test head 400 can be moved into positionedabove one of the holes 430 such that a central axis of the hole isapproximately coaxial with the piezoelectric sensing elements of thesensing element group 420. In some implementations, the mobile test head400 may have a positioning system that determines and notifies a userwhen the mobile test head is properly positioned over a hole. Whenproperly positioned over a hole 30 to be tested, the mobile test head400 is operable to activate the annular-shaped second piezoelectricsensing element 424 to generate an acoustic wave in the structure 412around the hole 30 to be tested, and receive input regarding thecharacteristics of the generated acoustic wave from the plurality offirst piezoelectric sensing elements 422. As described above, the rolesof the first piezoelectric sensing elements 422 and the secondpiezoelectric sensing element 424 can be reversed in some embodiments.The mobile test head 400, or a controller coupled to the test head, canbe equipped to compare the input received from the plurality of firstpiezoelectric sensing elements 422 with an expected or baseline input todetect the presence of damage in the structure 412 in a manner similarto that described above.

In the above description, certain terms may be used such as “up,”“down,” “upper,” “lower,” “horizontal,” “vertical,” “left,” “right,”“over,” “under” and the like. These terms are used, where applicable, toprovide some clarity of description when dealing with relativerelationships. But, these terms are not intended to imply absoluterelationships, positions, and/or orientations. For example, with respectto an object, an “upper” surface can become a “lower” surface simply byturning the object over. Nevertheless, it is still the same object.Further, the terms “including,” “comprising,” “having,” and variationsthereof mean “including but not limited to” unless expressly specifiedotherwise. An enumerated listing of items does not imply that any or allof the items are mutually exclusive and/or mutually inclusive, unlessexpressly specified otherwise. The terms “a,” “an,” and “the” also referto “one or more” unless expressly specified otherwise. Further, the term“plurality” can be defined as “at least two.”

Additionally, instances in this specification where one element is“coupled” to another element can include direct and indirect coupling.Direct coupling can be defined as one element coupled to and in somecontact with another element. Indirect coupling can be defined ascoupling between two elements not in direct contact with each other, buthaving one or more additional elements between the coupled elements.Further, as used herein, securing one element to another element caninclude direct securing and indirect securing. Additionally, as usedherein, “adjacent” does not necessarily denote contact. For example, oneelement can be adjacent another element without being in contact withthat element.

As used herein, the phrase “at least one of”, when used with a list ofitems, means different combinations of one or more of the listed itemsmay be used and only one of the items in the list may be needed. Theitem may be a particular object, thing, or category. In other words, “atleast one of” means any combination of items or number of items may beused from the list, but not all of the items in the list may berequired. For example, “at least one of item A, item B, and item C” maymean item A; item A and item B; item B; item A, item B, and item C; oritem B and item C. In some cases, “at least one of item A, item B, anditem C” may mean, for example, without limitation, two of item A, one ofitem B, and ten of item C; four of item B and seven of item C; or someother suitable combination.

As will be appreciated by one skilled in the art, aspects of the presentinvention can be embodied as a system, method, and/or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module,” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having program code embodied thereon.

Many of the functional units described in this specification have beenlabeled as modules, in order to more particularly emphasize theirimplementation independence. For example, a module may be implemented asa hardware circuit comprising custom VLSI circuits or gate arrays,off-the-shelf semiconductors such as logic chips, transistors, or otherdiscrete components. A module may also be implemented in programmablehardware devices such as field programmable gate arrays, programmablearray logic, programmable logic devices or the like.

Modules may also be implemented in software for execution by varioustypes of processors. An identified module of program code may, forinstance, comprise one or more physical or logical blocks of computerinstructions which may, for instance, be organized as an object,procedure, or function. Nevertheless, the executables of an identifiedmodule need not be physically located together, but may comprisedisparate instructions stored in different locations which, when joinedlogically together, comprise the module and achieve the stated purposefor the module.

Indeed, a module of program code may be a single instruction, or manyinstructions, and may even be distributed over several different codesegments, among different programs, and across several memory devices.Similarly, operational data may be identified and illustrated hereinwithin modules, and may be embodied in any suitable form and organizedwithin any suitable type of data structure. The operational data may becollected as a single data set, or may be distributed over differentlocations including over different storage devices, and may exist, atleast partially, merely as electronic signals on a system or network.Where a module or portions of a module are implemented in software, theprogram code may be stored and/or propagated on in one or more computerreadable medium(s).

The computer readable medium may be a tangible computer readable storagemedium storing the program code. The computer readable storage mediummay be, for example, but not limited to, an electronic, magnetic,optical, electromagnetic, infrared, holographic, micromechanical, orsemiconductor system, apparatus, or device, or any suitable combinationof the foregoing.

More specific examples of the computer readable storage medium mayinclude but are not limited to a portable computer diskette, a harddisk, a random access memory (RAM), a read-only memory (ROM), anerasable programmable read-only memory (EPROM or Flash memory), aportable compact disc read-only memory (CD-ROM), a digital versatiledisc (DVD), an optical storage device, a magnetic storage device, aholographic storage medium, a micromechanical storage device, or anysuitable combination of the foregoing. In the context of this document,a computer readable storage medium may be any tangible medium that cancontain, and/or store program code for use by and/or in connection withan instruction execution system, apparatus, or device.

The computer readable medium may also be a computer readable signalmedium. A computer readable signal medium may include a propagated datasignal with program code embodied therein, for example, in baseband oras part of a carrier wave. Such a propagated signal may take any of avariety of forms, including, but not limited to, electrical,electro-magnetic, magnetic, optical, or any suitable combinationthereof. A computer readable signal medium may be any computer readablemedium that is not a computer readable storage medium and that cancommunicate, propagate, or transport program code for use by or inconnection with an instruction execution system, apparatus, or device.Program code embodied on a computer readable signal medium may betransmitted using any appropriate medium, including but not limited towire-line, optical fiber, Radio Frequency (RF), or the like, or anysuitable combination of the foregoing

In one embodiment, the computer readable medium may comprise acombination of one or more computer readable storage mediums and one ormore computer readable signal mediums. For example, program code may beboth propagated as an electro-magnetic signal through a fiber opticcable for execution by a processor and stored on RAM storage device forexecution by the processor.

Program code for carrying out operations for aspects of the presentinvention may be written in any combination of one or more programminglanguages, including an object oriented programming language such asJava, Smalltalk, C++, PHP or the like and conventional proceduralprogramming languages, such as the “C” programming language or similarprogramming languages. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

The computer program product may be shared, simultaneously servingmultiple customers in a flexible, automated fashion. The computerprogram product may be standardized, requiring little customization andscalable, providing capacity on demand in a pay-as-you-go model.

The computer program product may be stored on a shared file systemaccessible from one or more servers. The computer program product may beexecuted via transactions that contain data and server processingrequests that use Central Processor Unit (CPU) units on the accessedserver. CPU units may be units of time such as minutes, seconds, hourson the central processor of the server. Additionally the accessed servermay make requests of other servers that require CPU units. CPU units arean example that represents but one measurement of use. Othermeasurements of use include but are not limited to network bandwidth,memory usage, storage usage, packet transfers, complete transactionsetc.

Aspects of the embodiments may be described above with reference toschematic flowchart diagrams and/or schematic block diagrams of methods,apparatuses, systems, and computer program products according toembodiments of the invention. It will be understood that each block ofthe schematic flowchart diagrams and/or schematic block diagrams, andcombinations of blocks in the schematic flowchart diagrams and/orschematic block diagrams, can be implemented by program code. Theprogram code may be provided to a processor of a general purposecomputer, special purpose computer, sequencer, or other programmabledata processing apparatus to produce a machine, such that theinstructions, which execute via the processor of the computer or otherprogrammable data processing apparatus, create means for implementingthe functions/acts specified in the schematic flowchart diagrams and/orschematic block diagrams block or blocks.

The program code may also be stored in a computer readable medium thatcan direct a computer, other programmable data processing apparatus, orother devices to function in a particular manner, such that theinstructions stored in the computer readable medium produce an articleof manufacture including instructions which implement the function/actspecified in the schematic flowchart diagrams and/or schematic blockdiagrams block or blocks.

The program code may also be loaded onto a computer, other programmabledata processing apparatus, or other devices to cause a series ofoperational steps to be performed on the computer, other programmableapparatus or other devices to produce a computer implemented processsuch that the program code which executed on the computer or otherprogrammable apparatus provide processes for implementing thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The schematic flowchart diagrams and/or schematic block diagrams in theFigures illustrate the architecture, functionality, and operation ofpossible implementations of apparatuses, systems, methods and computerprogram products according to various embodiments of the presentinvention. In this regard, each block in the schematic flowchartdiagrams and/or schematic block diagrams may represent a module,segment, or portion of code, which comprises one or more executableinstructions of the program code for implementing the specified logicalfunction(s).

It should also be noted that, in some alternative implementations, thefunctions noted in the block may occur out of the order noted in theFigures. For example, two blocks shown in succession may, in fact, beexecuted substantially concurrently, or the blocks may sometimes beexecuted in the reverse order, depending upon the functionalityinvolved. Other steps and methods may be conceived that are equivalentin function, logic, or effect to one or more blocks, or portionsthereof, of the illustrated Figures.

Although various arrow types and line types may be employed in theflowchart and/or block diagrams, they are understood not to limit thescope of the corresponding embodiments. Indeed, some arrows or otherconnectors may be used to indicate only the logical flow of the depictedembodiment. For instance, an arrow may indicate a waiting or monitoringperiod of unspecified duration between enumerated steps of the depictedembodiment. It will also be noted that each block of the block diagramsand/or flowchart diagrams, and combinations of blocks in the blockdiagrams and/or flowchart diagrams, can be implemented by specialpurpose hardware-based systems that perform the specified functions oracts, or combinations of special purpose hardware and program code.

The present subject matter may be embodied in other specific formswithout departing from its spirit or essential characteristics. Thedescribed embodiments are to be considered in all respects only asillustrative and not restrictive. All changes which come within themeaning and range of equivalency of the claims are to be embraced withintheir scope.

What is claimed is:
 1. An apparatus for detecting damage in a structure, comprising: a plurality of first piezoelectric sensing elements arranged in a generally circular shape; and an annular-shaped second piezoelectric sensing element positioned adjacent the plurality of first piezoelectric sensing elements; wherein one of the plurality of first piezoelectric sensing elements or the annular-shaped second piezoelectric sensing element generates a wave through the structure and other of the plurality of first piezoelectric sensing elements or the annular-shaped second piezoelectric sensing element senses the wave after passing through the structure.
 2. The apparatus of claim 1, wherein the annular-shaped second piezoelectric sensing element surrounds the plurality of first piezoelectric sensing elements.
 3. The apparatus of claim 1, further comprising: a plurality of third piezoelectric sensing elements arranged in a generally circular shape surrounding the annular-shaped second piezoelectric sensing element; and an annular-shaped fourth piezoelectric sensing element that surrounds the plurality of third piezoelectric sensing elements.
 4. The apparatus of claim 1, wherein the plurality of first piezoelectric sensing elements comprises at least five piezoelectric sensing elements.
 5. The apparatus of claim 1, wherein the plurality of first piezoelectric sensing elements and the annular-shaped second piezoelectric sensing element are bonded onto the structure
 6. The apparatus of claim 5, further comprising a plurality of electrical lead pairs bonded to the structure, wherein each of the plurality of electrical lead pairs comprises at least two electrical leads electrically coupled to a respective one of the first and second piezoelectric sensing elements.
 7. The apparatus of claim 1, further comprising a mobile test head movable along the structure, the mobile test head comprising the plurality of first piezoelectric sensing elements and the annular-shaped second piezoelectric sensing element.
 8. The apparatus of claim 1, wherein the plurality of first piezoelectric sensing elements and the annular-shaped second piezoelectric sensing element collectively define a first sensing element group, the apparatus further comprising a second sensing element group spaced apart from the first sensing element group, the second sensing element group comprising a plurality of third piezoelectric sensing elements arranged in a generally circular shape and an annular-shaped fourth piezoelectric sensing element positioned adjacent the plurality of third piezoelectric sensing elements, and wherein one of the plurality of first piezoelectric sensing elements and the annular-shaped second piezoelectric sensing element generates a wave through the structure and one of the plurality of third piezoelectric sensing elements and the annular-shaped fourth piezoelectric sensing element senses the wave after passing through the structure.
 9. The apparatus of claim 8, wherein the first sensing element group is centered around a first hole formed in the structure and the second sensing element group is centered around a second hole formed in the structure.
 10. The apparatus of claim 8, further comprising a controller configured to selectively switch between operation in a first mode and second mode, wherein in the first mode the controller activates the first sensing element group to generate a first wave and receives input from the first sensing element group regarding characteristics of the first wave and activates the second sensing element group to generate a second wave and receives input from the second sensing element group regarding characteristics of the second wave, and wherein in the second mode the controller activates the first sensing element group to generate a third wave and receives input from the second sensing element group regarding characteristics of the third wave.
 11. The apparatus of claim 1, further comprising a controller configured to activate the one of the plurality of first piezoelectric sensing elements or the annular-shaped second piezoelectric sensing element to generate the wave, and to receive input from other of the plurality of first piezoelectric sensing elements or the annular-shaped second piezoelectric sensing element that sense the wave after passing through the structure.
 12. The apparatus of claim 11, further comprising a plurality of electrical lead pairs bonded to the structure, wherein each of the plurality of electrical lead pairs comprises at least two electrical leads electrically coupled to a respective one of the first and second piezoelectric sensing elements, and wherein the controller comprises an electrical lead interface that interfaces with the plurality of electrical lead pairs to send electrical signals to or receive electrical signals from the plurality of first piezoelectric sensing elements and the annular-shaped second piezoelectric sensing element.
 13. The apparatus of claim 1, wherein the annular-shaped second piezoelectric sensing element is substantially coaxial with the generally circular shape of the plurality of first piezoelectric sensing elements.
 14. The apparatus of claim 13, wherein the annular-shaped second piezoelectric sensing element is positioned radially outwardly away from the plurality of first piezoelectric sensing elements.
 15. The apparatus of claim 1, wherein each of the first piezoelectric sensing elements comprises a piezoelectric sensor disc.
 16. A system, comprising: a structure comprising a hole; a plurality of first piezoelectric sensing elements arranged in a generally circular shape about the hole; and an annular-shaped second piezoelectric sensing element positioned adjacent the plurality of first piezoelectric sensing elements and about the hole; wherein one of the plurality of first piezoelectric sensing elements or the annular-shaped second piezoelectric sensing element generates a wave through the structure and other of the plurality of first piezoelectric sensing elements or the annular-shaped second piezoelectric sensing element senses the wave after passing through the structure adjacent the hole.
 17. The system of claim 16, wherein each of the plurality of first piezoelectric sensing elements is positioned an equal distance away from a center of the hole, and the annular-shaped piezoelectric sensing element is coaxial with the hole.
 18. The system of claim 16, wherein the structure comprises an aircraft.
 19. The system of claim 16, wherein the plurality of first piezoelectric sensing elements and the annular-shaped second piezoelectric sensing element collectively define a first sensing element group and the hole is a first hole, the structure further comprising a second hole spaced apart from the first hole, and the system further comprising a second sensing element group comprising a plurality of third piezoelectric sensing elements bonded to the structure and arranged in a generally circular shape about the second hole and an annular-shaped fourth piezoelectric sensing element bonded to the structure and being adjacent the plurality of third piezoelectric sensing elements, and wherein one of the plurality of first piezoelectric sensing elements and the annular-shaped second piezoelectric sensing element generates a wave through the structure and one of the plurality of third piezoelectric sensing elements and the annular-shaped fourth piezoelectric sensing element senses the wave after passing through the structure.
 20. A method for detecting damage in a structure, comprising: generating a wave through the structure from one of a plurality of first piezoelectric sensing elements arranged in a generally circular shape or an annular-shaped second piezoelectric sensing element; and sensing the wave after passing through the structure at other of the plurality of first piezoelectric sensing elements or the annular-shaped second piezoelectric sensing element. 